The present invention relates to a tool for measuring the pressure created in an oil well by the underground formation where the well is drilled, with the well being delimited by casing having production string installed therein, and with the string including a section that constitutes a sliding sleeve circulating valve or sliding side door (SSD), said valve being capable, on command, of putting the space inside the string into communication with the annular space lying between the string and the casing. This is done by bringing orifices through the wall of said section and through the sliding sleeve into register.
Pressure measurements in oil wells provide important information on the characteristics of the oil-bearing formations through which they are drilled. In wells where production is obtained by means of electric pumping, the pressure drop due to a sudden momentary increase in the flow rate at the surface can be used to calculate the production index, i.e. the production capacity of the well as a function of pressure drop. Given this index, which depends on the permeability and on the size of the reservoir constituted by the underground formation, it is possible to adjust the production flow rate to its optimum value.
It is also possible to set up an increase in pressure in a well by suddenly stopping pumping. The rate of this increase and its form as a function of time characterize the manner in which the reservoir responds and make it possible to evaluate its size and its porosity, to discover whether it is fractured, etc., thereby giving rise to a better description of the reservoir and to a more accurate understanding of its future capabilities and of the advisability of drilling other wells in its vicinity.
In a third application relating to wells which are exploited by electrical pumping, it is possible to evaluate the efficiency of the pump being used and to detect possible damage which may be shown up by abnormal variations in efficiency, by measuring the pressure created by the underground formation at a given depth, i.e. the pressure reigning in the above-mentioned annular space, while simultaneously measuring the pressure in the string at the same depth, which pressure depends on pumping characteristics.
The pressure created by the underground formation must be measured in the annular space lying between the casing and the production string. The height of the column of oil in said annular space is directly related to this pressure, and proposals have been made to measure this height from the go and return time of an acoustic wave emitted from the surface of the ground and reflected from the air-oil interface of the column. However, this procedure is not usable when the annular space is closed at the top by sealing means known as a packer.
Attempts have also been made to measure the pressure at the bottom of a well which is being exploited by electrical pumping by associating a pressure sensor with the pump. The results obtained in this way have been unsatisfactory since the pressure data provided by such a sensor (which must remain at the bottom of the well for as long as the pump remains at the bottom) falls off in quality over the years. In addition, the electrical signals delivered by the sensor are mixed with noise. As a result measuring accuracy is very mediocre.
Another proposal consists in placing a pressure gauge on the outside surface of the production string. However, the presence of said packer (i.e. sealing means) in the annular space interferes with passing an electrical cable connected to the pressure gauge since that cable must reach the surface by running up the annular space.